Densitometer



United States Patent [7 21 Inventor Dan Sibalis Brooklyn, New York [2]]App]. No. 703,100

[22] Filed Feb. 5, 1968 [45] Patented Nov. 24, 1970 [73] Assignee BerkeyPhoto, Inc.

New York, New York [54] DENSITOMETER 10 Claims, 4 Drawing Figs.

[52] US. Cl 356/202, 250/2 l7 [51] lnt.Cl G0ln 21/06; E04g 17/06 [50]Field ofSearch 356/23 l,

' [56] References Cited UNITED STATES PATENTS 3,354,773 I [1967 Shreve356/47 OTHER REFERENCES Colorimetry Albert A. Shurkus; Radio News, June1944.

Primary Examiner-Ronald L. Wibert Assistant Exam inerConrad ClarkAtlorney Edward T. Connors ABSTRACT: An instrument is disclosed formeasuring the reflectance or the density of an article, or either ofthese characteristics. Under operation to measure reflectance a variablelight source is positioned to illuminate the article, the light beingtransmitted to the article by an apertured mirror. Firstlight-responsive means is positioned so as to receive light reflectedfrom the article through the aperture. Control means is used to vary thelight source output to a predetermined value, the control means beingactuated by the first lightresponsive means. A second light-responsivemeans is positioned to receive light from the source and to controlindicating means the reading of which is related to the intensity oflight reflected from the article.

Under operation to measure density, the first light-responsive means ispositioned to receive light transmitted through the article and themeter indicates the intensity of light passed through the article.

In a construction to measure either reflectance or density a pair oflight-responsive means is provided, one being positioned to receivelight reflected through the apertured mirror, the other being positionedto receive light transmitted through the article, the pairoflight-responsive means being connected through a double throw switchfor alternative use.

I AMP p t nted Nov. 24,1970 v 3 5 7 I: 1: 2% AMP FIG. I

1 34- 37 L3 42 p 44 I l 3?, i \25 ,ggQ so -4| I I I FIG. 2

l f- DENSITOMETER The present invention relates to an instrument termedherein a densitometer and which is suitable for measuring the opticaldensity ofphotographic film orthe like or for measuring light reflectedfrom the surface'of an article such as a print. or from a surface onwhich is projected an image such as an enlarging easel. The densitometerthus may provide a measurement of density and reflectance, or eitherofthese charac teristics. I

Heretofore densitometers have been provided in which the light reflectedfrom, or transmitted through, the article has been measured by the useof light-responsive devices but the light source has been supplied bythe usual supply line. The

light sensor or light-responsive means is thus operated under I variouslevels of reflected or transmitted light thus leading to inaccuratemeasurements because of nonlinearity of the light sensors, or because ofpossible variations in the light source due to line voltage variationsor aging ofthe lamp.

The present invention aims to overcome the difficulties anddisadvantages of prior devices by providing a densitometer in which thelight sensor or light-responsive means is operated at a constant lightlevel.

In accordance with the invention this is accomplished by providing avariable light source with control means responsive to the reflected ortransmitted light so that the light on the light sensor is maintainedconstant by varying the light source output. A separatesensor isus'ed tomeasure the level of the light source which is read out by a meterappropriately calibrated to indicate reflectance or density values.

The construction in accordance with the invention is advantageous in"that the use of inexpensive less sensitive light for the densitometerfor use with an alternating current supply.

FIG. 3 is aschematic wiring diagram of an electrical circuit for thedensitometer for use with a direct current supply.

FIG. 4 is a schematic sectional drawing of the mechanical structureolthc densitometer.

Referring to the drawing there is shown in FIG. 1 a schematic drawing ofa densitometer 9 in accordance with the invention. The structure shownin FIG. 1 is solely for measuring the'optical density of an article 10such as a photographic film. A variable light source 11 is positioned soas to direct light to the film 10. On the opposite side of the film 10is a first lightresponsive device 12 which may be a silicon or seleniumphotocell or cadmium sulfide or selcnide photoresistors with its outputconnected. to an amplifier 13. The amplifier 13 may include controlmeans for varying the light source 11. Second light-responsive means 14may also be a silicon or selenium photocell which is connected tosuitable electric meter means 15 and a battery 16 so that the reading'ofthe meter 15 is relative to the intensity of the light source 11.

In the operation of the densitometer shown in FIG. 1 light from thesource 11 is passed through the film 10 and attenuated somewhatdepending upon the optical density of the film 10. The attenuated lightis impinged upon the first light cell 12;

and by means of a suitable amplifier 13 caused to raise the intensity ofthe light source 11' so that the attenuated light reaching the firstlight cell 12 is the same as in theabsence of the film 10. Thus thephotocell 12 always receives the same.

amount of light (self-nulling). Therefore any suitable light cell may beused even one with a nonlinear response. The raise in intensity of thelight source 11 is sensed by the photocell 14 and the magnitude of theincrease in intensity is shown by the reading ofthe meter 15.

' In FIG. 2 there is shown a schematic diagram of a suitable circuit 20for the densitometer and which may be supplied from an alternatingcurrent supply 21 through terminals 22 i and 23. The lamp 11 may be ofthe incandescent type and is controlled to produce a variable lightoutput. Basically the lamp control circuit is an extended. range fullwave phase control circuit in which the photocell 12 controls the phaseangle of triggering of a silicon controlled rectifier 25. This circuitis generally that-described in the General Electric SCR Manual. fourthedition,o'n pages l9l 193. The lamp 11 is connected across the a.c.line22-23 in series with the bridge rectifier 26-29 thus permittingtriggering on both halves of the a.c. cycle. The SCR 25 is connectedacross the bridge to control the light output of the lamp 11'.Triggering of the SCR 12 is performed by unijunction transistor 30having its Bl connected as an emitter follower for the control electrodeof the SCR 25. B2 of the unijunction transistor 30 is connected to aclamped waveform supply provided by zener diodcs'31 and '32. The zener32 has a slightly lower zener voltage than zener 31 in order to hold thetop of the clamped waveform flat. Re sistor 34 in series with thephotocell 12 forms a voltage divider which through diode 35 determinesthe charge on capacitor 36. Charging resistor 37 is provided toinitially set system gain. Resistors 39 and 40 are voltage droppingresistors. Resistor 41 is used as a gate resistor and resistor 42 is abase-stabilizing resistor for the unijunction transistor 30. The circuit20 of FIG.

2 may also include a third photocell 1211 connected for alternateoperation with the first photocell 12 by means of a dou ble'throw switch44.The use of the third photocell will be described in the operation ofthe densitometer shown in FIG.

In the operation of the circuit 20 of FIG. 2 a variation in lightimpinging onthe photocell 12 controls the angle of triggering'ofthe SCRZS and thus the light'output ofthe lamp 11. The second photocell l4 andmeter 15 of the circuit 20 operate as in the circuit 9 of FIG. 1 toprovide an indication of the light output of the lamp 11.

In FIG. 3 there is shown a schematic diagram of another circuit for thedensitometer and which may be supplied from a direct current supplythrough terminals 48 and 49. In the circuit 50 'the lamp 11 iscontrolled by the switching of a transistor 51 controlled by aDarlington connected amplifier including transistors 52 and 53. Resistor54 is a collector load for the transistor 52. Resistor 55 connected inseries with the photocell 12 providesa voltage divider for the controlof the Darlington amplifier. Ligl1t variations sensed by the photocelll2 vary the resistance thereofand thus the potential applied to thebaseof the transistor 53, the end result being the control oflightoutput ol'the lamp 11.

A suitable mechanical structure for the densitometer is shown in FIG. 4and includes a housing having a lighttight v compartment 61 for the lamp11 supported by a lamp base 62.

t A lens system 65 directs light through a filter system 66 to ali'ghtreflected from an article in the supporting means 71 to impingeupon the third photocell 12a.

The housing 60 extends upwardly to provide a chamber 74 in which ispositioned the photocell 12. At any convenient place within the housingis located the electronic circuitry indicated by the block 75. At theupper end of the housing 60 is the meter 15 preferably positioned at anangle as indicated for ease in reading. I

The densitometer may be arranged to provide either reflectance ordensity readings or both type readings. In using the densitometer forreflectance readings the double-throw switch 44 is moved to the positionto connect the photocell 12a in the circuit. In this case light from thelamp 1!. is passed through the lens 65, the filter 66, reflected by themirror 67 to the article in the supporting means 71. Reflected lightfrom the article is directed through the aperture 72 to the photocell12a. Control is had of the light output and the increased intensity ofthe light readon the meter 15 calibrated to show the reflectance ofthearticle.

in using the densitometer for density readings the doublethrow switch 44is moved into position to connect the photocell 12 into the circuit anddisconnect the photocell 12a. The reading of the meter 15 will then berelative to light transmitted through the article.

From the foregoing description it will be seen that a densitometer isprovided capable ofmaking reflectance or density measurements. Obviouslyby omitting certain parts the device may be constructed for makingeither reflectance or density readings.

While the invention hasbeen described and illustrated with reference tospecific embodiments thereof, it will be understood that otherembodiments may be resorted to without departing from the invention.Therefore, the form of the invention set out above should be consideredas illustrative and not as limiting the scope of the following claims.

I claim: o 1

l. A densitometer comprising supporting means for an article to besubjected to light so that the article provides a field of view, apossibly variable light source for'illuminating said article, firstlight-responsive means actuated by light fromsaid field of view, controlmeans for said possiblyvariable light source actuated by said firstlight-responsive means so that the light output is ltept constant,second light-responsive means actuated by light from said light source,and indicating means for said second light-responsive means whereby areading of said indicating means is related to the intensity of lightfrom said field ofview.

2. A densitometer according to claim 1 in which said supporting means isa film holder.

3. A densitometer according to claim 1 in which said firstlight-responsive means is actuated by light passed through said article.i

4. A densitometer according to claim 1 in which said firstlight-responsive means is actuated by light reflected from said article.

5. A densitometer according to claim 1 in which said control meansincludes'a'n amplifier, and output means for the amplifier beingconnected to control said possibly variable light source sothat thelight output is kept constant.

- 6. A, densitometer according to claim 1 in which said indicating meansis an electric meter. 1

7. A densitometer according to claim 1 in which a mirror is provided andpositioned to reflect light from said variable light source to saidarticle, said first light-responsive means being positioned on one sideof said article and said mirror being positioned on the other side ofsaid article so that said first light-responsive means is actuated bylight passed through said article.

8. A densitometer according to claim 1 in which an apertured mirror isprovided and positioned to reflect light from said variable light sourceto said article, said first light-responsive means being positionedbehind said mirror in alinement with said article through said apertureso that said first lightresponsive means is actuated by light reflectedfrom said article.

9. A densitometer according to claim 1 in which an apertured mirror isprovided, and a third light-responsive means is provided-and positionedin alinement with said article through said mirror aperture so'that.said third light-responsive means is actuated by light reflected fromsaid article, and dual position switching means is provided operative toselectively connect said first or said third light-responsive means tosaid control means.

, 10. A densitometer comprising a film holder adapted to receive a film,a possibly variable light source for illuminating a film in said filmholder, first light-responsive means positioned to receive lighttransmitted through a film in said film holder, amplifier means for theoutpu of said first lightresponsive means, control means for saidvariable light source actuated by the output of said amplifier so thatthe light output is keptconstan't, second lightresponsive means actuatedby light from said light source, and electric meter means con nected tosaid second light-responsive means whereby a reading of said meter isrelated to the intensity of light passed through said film.

